1. Field of the Invention
The present invention relates to a liquid crystal display device and a method of manufacturing the same, and particularly to a transflective liquid crystal display device (hereinafter referred to as a transflective LCD device) and a method of manufacturing the same.
2. Description of the Related Art
Transflective LCD devices are increasingly adopted to displays or display units which are mounted on mobile devices such as recent cellular phones, digital camcorders and digital cameras. This is because usual transmissive liquid crystal display devices (hereinafter, referred to as a transmissive LCD device), which are used for a display of a laptop computer and a monitor of a desktop computer, have the following disadvantage. Specifically, in a case of the transmissive LCD device, it is hard to view the display at a site in strong sunlight due to the reflection of the light on the surface of the LCD device.
As a light source, the use the backlight indoors, and use ambient light, such as the sunlight, outdoors. Thus, as for transflective LCD devices, a good display performance is obtained anywhere. In addition, since the power consumption in the transflective LCD devices is small, the devices are most suitable as the displays for the mobile devices and the cameras.
Recent mobile devices, especially cellular phones and the like, have functions not only as telephones, but also as digital cameras and as devices that allow the users to watch TV. In such uses, the LCD devices have been more frequently used in a landscape mode in addition to a portrait mode. In such a case, when a viewing angle (a field of view) of an LCD device is narrow, the difference in display quality between the portrait mode and the landscape mode is markedly great. As a result, the performance of the LCD device decreases in the case of the landscape mode. Consequently, demand for a small LCD-device with a wide viewing angle has increased.
Some of methods for making a transflective LCD device to have a wide viewing angle will be described below. In general, an LCD device having twisted nematic type (TN type) liquid crystal material has a narrow viewing angle. For this reason, an optical compensation film is frequently used as a method of increasing the viewing angle. In this method, although the viewing angle in the horizontal direction increases to some extent, an increase in the viewing angle in the vertical direction is limited. As a result, the viewing angle in the portrait mode and that in the landscape mode are different from each other in this LCD device. This is not a problem in a case of a display device, such as a monitor, which is fixed when in use. However, in a case of a mobile device used in both portrait and landscape modes, the performance of the LCD device may decrease for a TN-type LCD device in which the optical compensation film is used. As a consequence, in recent years, a vertical-alignment liquid crystal display device (a VA-LCD device) having liquid crystal material with negative dielectric anisotropy is increasingly used for a mobile device. In a case of a VA-LCD device, when a voltage is not applied, liquid crystal molecules (LC molecules) are aligned perpendicular to a glass substrate. When a voltage is applied, the LC molecules are tilted according to a voltage value, and then the LC molecules show optical anisotropy. However, when the LC molecules are not regulated, the directions in which the LC molecules incline are not fixed. For this reason, it is necessary to regulate the inclination directions of the LC molecules. When regulating the directions, the property of a wide viewing angle can be obtained by using a multi-domain alignment method for causing the LC molecules to evenly incline in all directions. The alignment of the LC molecules is usually divided into two or four directions.
In a usual multi-domain alignment method of the LC molecules, each electric flux line between substrates facing each other is deformed so as to control the inclination directions of the LC molecules. There are two methods for deforming the electric flux lines between substrates, and thereby controlling the inclination directions of the LC molecules. In one method, a protrusion is formed of an organic insulating film or the like. In the other method, slits are provided to a pixel electrode. An example of the method in which a protrusion is formed of an organic insulating film or the like is disclosed in Japanese Patent Application Laid-open Publication No. H-11-242225 (hereinafter referred to as Patent Document 1). An example of the method in which slits are provided to a pixel electrode is disclosed in Japanese Patent Application Laid-open Publication No. 2004-069767 (hereinafter referred to as Patent Document 2). Patent Documents 1 and 2 described above are used separately or in combination. Manufacturing costs for multi-domain alignment of the LC molecules by using the protrusion is higher than that using the electrode slit since an organic insulating film needs to be used to form the protrusion. Hence, in many cases, the protrusion is provided to one of a pair of substrates forming an LCD device, and slits are provided to a pixel in the other substrate. The use of any one of the methods increases the number of processes for manufacturing a VA-LCD device as compared with a case where TN-type LC is used, and thereby manufacturing costs for the VA-LCD increases.
An example, in which neither a protrusion or slits are used for multi-domain alignment of the LC molecules, has been proposed in Japanese Patent Application Laid-open Publication No. 2003-287754 (hereinafter, referred to as Patent Document 3) and the like. This Patent Document 3 discloses a method in which multi-domain alignment of the LC molecules is achieved with a pixel electrode formed smaller than a common electrode facing the pixel electrode, and formed in a fairly-symmetric shape. However, there is a problem that the alignment of the LC molecules is not stable since the central point of each of multi-domains of LC molecules cannot be fixed.
Furthermore, in the transflective LCD device, conditions (retardation values) under which the optimum optical properties can be obtained are different between the transmissive and the reflective region not only in a case of the VA type LCD device, but also in a case of the other types. For this reason, as described in Japanese Patent Application Laid-open Publication No. H-11-242226 (hereinafter referred to as Patent Document 4), it is necessary to use a multi-gap structure in which the thickness of liquid crystal layer in the transmissive region is different from that of the reflective region. In addition, for purpose of obtaining the good reflective performance of transflective LCD device, it is necessary to form a reflective electrode with an uneven surface for obtaining a diffuse reflection property. To form the uneven surface, a photosensitive organic film is normally used. Aforementioned Patent Document 4 discloses a method in which the organic film for forming the uneven surface is also used as an adjusting layer for adjusting the thickness of an LC layer for forming a multi-gap structure.
As described above, in one of the pair of substrates in the VA type transflective LCD device, it is necessary to form three elements, that is, the adjusting layer for adjusting the thickness of the LC layer, the reflective electrode with the uneven surface, and any one of the protrusion and the pixel slit for multi-domain alignment of the LC molecules. Japanese Patent Application Laid-open Publication No. 2004-246328 (hereinafter referred to as Patent Document 5) discloses a method for concurrently forming protrusions and an organic insulating film with an uneven surface for diffusion reflection. Here, the protrusions are used to achieve multi-division alignment of the LC molecules. However, in the method of Patent Document 5, an organic insulating film needs to be formed twice in order to separately form an adjusting layer for the thickness of an LC layer.